The present invention relates to oscillator and transmitter circuits, and more particularly to the suppression of radiated harmonics from such oscillator and transmitter circuits by use of a narrow band filter in the feedback path at the effective antenna.
All oscillators and transmitters radiate energy on a fundamental, or center, frequency as well as at harmonic frequencies. For applications requiring high orders of stability piezoelectric crystals have been used as the oscillating element in lieu of LC tank circuits. However for most applications LC tank circuits are adequate. One application for oscillators and transmitters is in security alarm devices and garage door radio controls. These devices are designed for minimum cost and typically use a single transistor stage that relies on an exposed (unshielded) construction on a printed circuit board to radiate a coded signal. Since there has been no definable antenna or "point of emission" established by this open design technique, it has been difficult for the manufacturer of these devices to filter harmonic levels.
A typical oscillator and transmitter, such as the Model 50 Remote Control Oscillator manufactured by Chamberlain Manufacturing Corporation of Elmhurst, Ill., is shown in FIG. 1. This device oscillates because the gain of the transistor stage is sufficient at the desired frequency, such as 390 MHz, to allow positive feedback between the base and emitter. The collector is parallel tuned to the desired frequency to produce a high impedance and the emitter is series tuned to the desired frequency to provide a low impedance. The ratio of collector to emitter impedance sets the gain. The feedback loop goes from the base through a bypass capacitor C4 to signal ground, or negative battery return, through the emitter capacitor C3 that series resonates with the emitter inductance at the desired frequency. If either the bypass or emitter capacitors, C4 or C3, are removed, the feedback loop is opened and the transistor stage is stable and does not oscillate. If the value of the emitter capacitor C3 is altered sufficiently or the collector LC circuit, L, C1 and C2, is detuned, the frequency changes, the gain drops and the oscillator operates at a lower power level or quits oscillating. Thus, the typical transistor stage oscillator and transmitter is solely dependent on the active device, the biasing, and the values of the LC components in the circuit for operational performance, i.e., power level, frequency stability and frequency settability.
As shown in FIG. 2 the resulting output from this type of oscillator and transmitter radiates energy not only at the desired frequency, but also radiates significant energy at the harmonics of the desired frequency. In 1982 the Federal Communications Commission (FCC) adopted amendments to the rules in Part 15, Subpart E, which imposed limits of 125 microvolts per meter on the harmonic emissions over one thousand MHz for such oscillator and transmitter circuits as used in security alarm devices and garage door radio controls. Standard filtering techniques used to eliminate these harmonics are expensive compared to the total cost of the oscillator and transmitter. Therefore, what is desired is an effective and inexpensive method for suppressing radiated harmonics from such devices while maintaining oscillator frequency stability.